1. Field of the Invention
The present invention relates to an image projection apparatus and, more particularly, to an image projection apparatus using a transmission or reflection image modulation means such as a liquid crystal element.
2. Related Background Art
As a conventional image projection apparatus, one which projects an image using a liquid crystal element or DMD element as a light-transmission or light-reflection image modulation means is presently available.
A recent data grade projector, which is connected to a PC for use, has a larger number of display pixels for VGA (640xc3x97480), SVGA (800xc3x97600), XGA (1,024xc3x97768), and SXGA (1,280xc3x971,024). To the contrary, there is a tendency to reduce the element chip size from 1.3xe2x80x3 to 0.9xe2x80x3 or 0.7xe2x80x3. Therefore, the pixels of the image modulation means are laid out at a higher density and a smaller pitch.
Also, a so-called AV grade (for TV) image projection apparatus requires a larger number of display pixels in order to cope with EDTV and HDTV, and the image modulation means is decreasing in pitch.
In general, the light valve unit of the image modulation means has an external surface at a position offset by a light-transmitting member such as a cover glass or a substrate constituting the element itself. Upon image projection, dust attaching to the external surface forms a shadow defocused by the offset amount on the image to degrade the image quality.
Assuming that the image projection size and element size are constant, an increase in the number of pixels increases the relative ratio of dust to the pixel size, and dust of the same size affects a larger number of pixels.
Assuming that the image projection size and the number of pixels are constant, reduction of the element size increases the projection magnification, and the dust size also relatively increases by the magnification increase. Thus, the relative ratio of dust to the pixel size increases, and dust of the same size affects a larger number of pixels. That is, even smaller dust must be removed, which requires high internal cleanness of the apparatus.
Along with efforts to reduce the element size, efforts to improve the brightness of the image projection apparatus are also being made. However, these efforts result in an increase in the luminous flux surface density on the light valve area.
As a result, the element performance and service life are readily affected by optical energy converted into heat that is not effectively used for illumination owing to absorption by non-aperture (non-transmitting and non-reflecting) portions such as the driving circuit and mask layer of the element, and unavoidable absorption by optical element transmission layers as their transmittance is not 100% (e.g., a transparent electrode, a glass itself, and a polarizing plate which may serve as an indirect heat source because it is arranged near the element).
To prevent the adverse influence of ineffective illumination light, the image modulation means is air-cooled by wind blown from an electric fan. At this time, in order to reduce dust attaching to the external surface exposed to the illumination optical path of the image modulation means, an air filter is generally inserted in the wind path of the fan to maintain the internal cleanness of the apparatus.
Japanese Laid-Open Utility Model Application No. 6-002337 (liquid crystal cooler for liquid crystal projector) discloses a technique of incorporating no air filter but a liquid crystal light valve and convection fan in a closed box in which an optical path is ensured, transmitting heat to a convection medium while blocking dust, and externally dissipating heat of the medium by a cooling unit made up of a heat dissipation fan and electronic cooling (Peltier) element.
As described above, when the image modulation means of the image projection apparatus is air-cooled while reducing the pixel size, the apparatus must adopt a dustproof means for removing dust larger than a predetermined size without increasing the inlet/outlet resistance.
However, no prior art disclose an air filter optimization method or a setting method about the pixel size and air filter mesh pitch/absolute aperture size, which can be widely applied to products in this field, though the optical arrangement, the screen area size of the light valve, and the like are different between apparatuses.
For this reason, in developing an image projection apparatus using new optical components including a light valve, an image is actually projected by a sample apparatus to allow dust to attach, and filters are exchanged at various intervals while checking the image quality. However, the reliability of such test results and the working efficiency in these tests are low.
Further, the absolute aperture size must be xe2x80x9ca minimum necessary sizexe2x80x9d. A filter having an undesirably small aperture size or a filter having many apertures smaller than the necessary aperture size exhibits a light wind resistance, so the filter area must be unpreferably increased. This poses problems on the internal space distribution and cost of the apparatus.
Sealing the image modulation means in a box, like another prior art, seems to be an adequate dustproof measure theoretically. However, this arrangement requires driving power for the internal fan and electronic cooling element (power larger than generation energy of heat to be dissipated for cooling efficiency is consumed for medium cooling) while transferring heat from the inside to outside of the box. To dissipate heat outside the closed box, an apparatus means and power equivalent to direct air-cooling are required, resulting in low efficiency and high cost. Also, the box cannot be made of a general resin material having low thermal conductivity and low cost, so the box also increases the cost. Hence, this technique cannot be applied to a consumer product which receives power from a general power supply.
The objective of use of a closed box in the apparatus is to increase the defocusing distance of the dust-attaching surface by a medium free from any dust. However, dust still attaches to the incident/exit surface of a light-transmitting member forming an optical path in the box. General small dust hardly influences an image, but large dust such as cotton dust may influence an image. In general, evaluation of a degraded image portion (a defocused image by a dust shadow) on the screen at the same area ratio varies depending on the image definition. The defocused state changes in accordance with the angular apertures of the illumination and projection systems. That is, the defocused degree/state and a necessary dustproof level are important. The apparatus using a closed box has a box size (defocusing amount) at which small dust that cannot be removed by a general low-cost filter is not present, i.e., it has an optimum minimum size. The box and apparatus must be downsized in consideration of this optimum minimum size. However, no prior art considers this.
It is an object of the present invention to provide an image projection apparatus capable of quickly and precisely selecting a dustproof mechanism necessary for an image projection apparatus in which an image modulation means is air-cooled without sealing it in a box.
According to the present invention, there is provided an image projection apparatus capable of quantifying the relative relationship between, as variables, at least the size of an optical arrangement including the image modulation means, an effective F-number (the effective illumination angle of an illumination system/NA) including the F-number of a projection lens, the pixel pitch, the thickness of a liquid crystal glass, the refractive index of the glass, and the dust size, and optimizing with high efficiency a dust removal means widely applicable to products in this field.
It is still another object of the present invention to provide an image projection apparatus capable of avoiding an unwanted cost increase of an air filter and an undesirable load on a cooling system which are applied to products in this field, and improving the reliability and image quality of products in this field without increasing the apparatus size.
To achieve the above object, according to the present invention, there is provided an image projection apparatus comprising a display device which has a plurality of pixel apertures and displays image information, an illumination optical unit for illuminating the display device, a projection optical unit for projecting the image information displayed on the display device onto a predetermined surface, and a blower for blowing air to an atmosphere in the display device, wherein letting 2A be a pixel aperture width of the display device, xcex1 be an effective illumination angle of a projection optical system including the illumination optical unit and a projection optical unit in a medium defining a dust-attaching surface of the display device, t be a thickness of the medium defining the dust-attaching surface, and 2xcex4 be a dust diameter, a dustproof filter for preventing passage of dust having a size given by
xcex4xe2x89xa7{square root over ({D+L xc2x7(A+t+L xc2x7tan xcex1)2+L })} (for 0.03 less than D less than 0.3)xe2x80x83xe2x80x83(1)
is arranged on an inlet side of the blower.
In addition, according to the present invention, there is provided an image projection apparatus comprising at least one image display device, illumination means for illuminating the image display device, projection optical means for projecting an image displayed on the image display device, cooling means for cooling the image display device, and filtering means which has apertures for filtering air discharged by the cooling means to the image display device, and preventing entrance of dust having such a size as to generate a small region or black spot having a color discontinuous and different from a neighboring color on an image projected on a virtual screen.
The above and other objects, features, and advantages of the present invention will be apparent from the following description in conjunction with the accompanying drawings.